While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322–334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the organic EL element encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. The interface between the two layers provides an efficient site for the recombination of the injected hole/electron pair and the resultant electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610–3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material-dopant, which results in an efficiency improvement and allows color tuning.
Since these early inventions, further improvements in device materials have resulted in improved performance in attributes such as operational lifetime, color, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569; 5,409,783; 5,554,450; 5,593,788; 5,683,823; 5,908,581; 5,928,802; 6,020,078; and 6,208,077.
Notwithstanding these developments, there are continuing needs for organic EL device components that will provide better performance and, particularly, long operational lifetimes. It is well known that, during operation of OLED device, it undergoes degradation, which causes light output at a constant current to decrease. This degradation is caused primarily by current passing through the device, compounded by contributions from the environmental factors such as temperature, humidity, presence of oxidants, etc. However, for practical applications such as display, light output of an OLED device is expected to be nearly constant during useful lifetime of the display. In principle, aging can be compensated by passing more current through the device so that the light output is kept constant. Several methods have been described for adjusting of a current to compensate for device aging. Specifically, WO 99/41732, issued Aug. 19, 1999 to D. L. Matthies et al., included measurement of accumulated driving current as a method to adjust driving current corresponding to a constant luminance. This technique is based on the findings of Steven A. VanSlyke et al. [J. Appl. Phys. 69 (1996) 2160] who reported that the extent of device degradation is dependent on the charge transferred through the device, which is equivalent to accumulated current. However, due to the influence of environmental factors, such as temperature, accumulated current may not be a sufficiently good predictor of OLED device degradation. In above-identified WO 99/41732, as well as in U.S. Pat. Nos. 6,081,073 and 6,320,325, compensation for OLED device degradation is performed by means of utilizing light sensors that are optically coupled to an OLED device. Such methods are complex and can be expensive to implement because they require optically coupled sensors as well as additional electronic circuitry.
There is a need therefore for an improved method of detection of the extent of OLED device aging and compensating for it.